The concept of using an electromagnetic transmission to transfer power from one rotating shaft to another was proposed over a half century ago. Through the years, this type of transmission has been refined to provide better control of the torque ratio and more efficient operation; but, despite these efforts, the electromagnetic transmission has seen only limited practical use.
Almost without exception, typical electromagnetic transmissions include two rotating armatures whose shafts are connected in some fashion to one or more planetary gear trains. One of these armatures, referred to as the primary armature, is also rigidly connected to the power input shaft. The other armature, referred to as the secondary armature, is neither necessarily nor usually connected directly to the power output shaft of the transmission.
The physical makeup of such electromagnetic transmissions usually incorporates the two rotating armatures mounted adjacent to one another on concentric shafts, and a single stationary field structure supplied with current from an external source or from the generating armature. The output shaft of such a transmission is rigidly connected to an element of the planetary gear train such that its rotation will be some linear combination of the rotational velocities of the two armatures. In the low end of the velocity range (which may include negative velocities), the secondary armature acts as a generator supplying direct current to the primary armature which acts as a motor. At higher speeds, the roles of the two armatures are reversed and the primary armature, then acting as a generator, supplies direct current to the secondary armature, then acting as a motor.
The ratio of output shaft torque to input shaft torque is determined by the direction and magnitude of current flow between the primary and secondary armatures. To establish the correct flow of current from one armature to the other, one or more sets of movable commutator brushes are employed which allow the induced voltage of either or both armatures to be controlled within limits determined by the rotational velocities of the armatures. By rotating the brushes about the armature axis away from what is known as the quadrature-axis, the induced emf of a rotating armature can be reduced from a maximum. The voltage control of the armatures, in turn, dictates the current flow between the armatures.
The chief disadvantages of electromagnetic transmissions of the continuously variable torque converter type discussed above are, for the most part, related to their complexity as compared with, for example, fluid torque converters with their associated gearing. Probably the single most costly and troublesome component of even modern versions of such electromagnetic transmissions is the movable brush assembly and associated controller. Noteworthy in this respect are the limitations placed on the power rating of a DC machine by the commutating process. If the voltage across the brush terminals is too great, excessive brush and commutator wear can result.